Transmission of light sequences by frequency variation



Oct. 16, 1934. J. H. HAMMOND, .JR 1,977,438

TRANsMssIoNj oF 'LIGHT 'sEQuENcEs BY FREQENCY VARIATION Filed May 18, 1929 hh INVENTOR JOHN HMS HAMMOND JR- 7 wf@ l Q JLU-0L lllll- ATTORNEY Patented Oct. 16, 1934 UNITED STATES PATENT OFFICE TRANSMISSION OF LIGHT SEQUENCES BY FREQUENCY VARIATION Hitherto signals for light intensity transmission as exemplified in radio photographic and radio television transmission have been conveyed by amplitude variations in the radio transmis- 6 sion. This method of transmission is subject to the drawback, as is well known, that with constant emission at the transmitter, the received antenna current varies due to the fading phenomena in the intervening media.

Accordingly, in transmitting by amplitude modulation as in the usual processes, there will be an amplitude modulated current received at the receiver, but no accurate relationship will exist bel tween such current and the modulation at the i6 transmitter, and no possible method has been devised whereby the receiver mechanism may differentiate between the amplitude variations at the transmitter and those caused by the phenomena in the intervening media.

It is an object of the present invention to provide a system of transmission of varying light and sound intensities whereby this drawback in the systems at present in use may be overcome.

It is a further object of this invention to provide a radio transmission and receiving -system whereby signals may be sent for signal purposes with freedom from fading.

It is a still further object of this invention to provide a transmission and receiving system whereby different gradations of light intensities may be sent from one point to another in sequence at high speeds with freedom from distortion due to fading.

These and other objects will become apparent from the following specification taken in connection with the appended drawing.

The basic principle of the present invention is that whereas the intervening media may cause amplitude variations, it doesnot cause variations in the frequency of the signal between the transmission and reception. Accordingly, for example, if the transmitter can be arranged to send out any of ve distinct carrier frequencies, as, say, 10,000 kc.; 10,002 kc.; 10,004 kc.; 10,006 kc.; 10,008 kc., these can be used for conveying to the receiver electrical information that the receiver should register zero, weak, medium, strong, and intense signals to correspond therewith respectively. With this method, the receiver cannot improperly interpret the intensity of the signal transmitted because, as stated above, the intervening media cannot affect that characteristic of the signal which indicates the signal strength, that is the frequency of the transmision.

Having thus briefly described my invention, at`

tention is called tol the accompanying drawing in which:

Fig. 1 is a transmitter for transmitting relatively slow variations of light intensity;

Fig. 2 is a transmitter 4for transmitting high 60 frequency light variations; and,

Fig. 3 is a lreceiver 'for cooperation with the transmitter of either Fig. 1 or Fig. 2, and which is adapted to reproduce the varying light intensities of the respective transmitters.

Attention is more particularly invited to Fig. 1 which, as above stated, is suitable for transmission of slowly varying light sequences, as, for instance, facsimile transmission. In this figure light of varying intensities is brought to focus by the lens 1 upon the photoelectric cell 2. The light falling upon the photoelectric cell 2 emanates from the varying parts of the facsimile to be transmitted by any of the usual and well known methods. In series with the cell 2 is the battery 'I5 3 and the resistor 4 and in consequence of the variation in light intensity .a current varying to correspond with said variations of light intensity flows across the said resistor. This causes a variable voltage to be impressed across the grid fila- 8l) ment terminals of vacuum tube 5 which latter is of the usual and well known type arranged to act as a modulator. Into the plate circuit of the tube 5 is supplied a relatively high frequency voltage from a local source through the transformer 6 and a coupling resistor 7. The local source supplies energy to be modulated by the voltage impressed on the grid of 5 due to light variations. Since these variations across resistor 4 in some cases may be subaudible, or in any event 90 low frequency, an indirect method of amplification has been shown which controls the ow 'of energy from oscillator 6 through condenser 8 to resistor 9. Thus, tube 5 is a variable impedance, with the impedance changed in accordance with light values, with the result that a current of the local source is impressed on tube 10, modified in amplitude in accordance with the light signals. Tube 10, of course, represents any amplifier capable of handling frequencies in the range of the oscillation supplied through 6. After amplification, the frequency is demodulated by tube 12 to give variations in the capacity 15 in accordance with the light variations.

The high frequency supplied through the transformer 6 is so chosen as to permit easy amplification of the modulated signals, and may be of the order of 10,000 cycles. This amplification is produced in the thermionic device 10 in the output of which is connected the output transformer 11. 110

. detected current are bypassed by the condenser 13 and the low frequency pulsating currents are fed through the coil 14, producing in said coil a varying flux corresponding to the original variations in light intensity. This variation in iiux intensity is caused to produce a corresponding mechanical motion of the small grounded plate condenser 15 in the well known manner. The fixed plate 16 and the movable plate 15 constitute a variable condenser which forms a part of the circuit of a high frequency oscillator 17. The frequency of the energy generated by said high frequency oscillator is determined chiefly by the constants of condenser 18 and the coil 19, but is modified by the capacityvvariations of the variable condenser composedof the plates 15 and 16. Means are provided to radiate the varying frequency oscillations produced by the varying oscillatory circuit which means transmits the varying frequency oscillations by any of the usual and well known methods.

Attention is now more particularly invited to Fig. 2 which showsa modified transmitter which is suitable for more faithful transmissions and for a wider range of speeds of light variations. In this arrangement the pulsations of voltage across a grid resistance 20, produced by a photoelectric cell iii/fthe same manner as that in Fig. 1, are amplified' directly by the resistance coupled amplifier devices 21, 22, and 23. This resistance coupled amplifier is adapted to amplify in a uniform manner irrespective of the frequency of the light variations. The amplifier 23 produces a varying voltage across the resistor 24. The resistor 24 is tapped and adapted to supply a pulsating voltage simultaneously to the grid and plate circuits of the device 25, which latter, with its associated circuits, forms a high frequency oscillator. By the suitable adjustment of the tap, it has been found that the combined voltage variations of the grid and plate variations will produce variations in frequency of the oscillations without amplitude changes as has been clearly set forth in Patent No. 1,599,586 of October 14, 1926, to E. S. Purington. This device operates at exceedingly high speeds and for high rates of intensity variation is much more suited to vary the frequency than the mechanically operated device of Fig. 1. The energy produced by the tube 25 and controlled as to frequency by the amplified variations, is then amplified by the device 26 and its associated circuits. The energy thus amplified may be radiated or transmitted in the usual manner as, for instance, from an antenna.

Referring more particularly to Fig. 3, there is shown a receiver for cooperation with the transmitter shown in either Fig. 1 or Fig. 2. This receiver includes a broadly tuned input circuit coupled by means of the transformer 27 to the broadly tuned grid circuit of the detector device 28. These circuits are so constructed as to respond with practically equal intensity to all of the incoming frequencies. A generator 29 is adapted to supply a high frequency which is approximately the same as that employed in the The beat detector action caused when the local oscillator is suitably adjusted causes an intermediate frequency to be produced which varies between limits in accordance with the frequency of the incoming signal. For example, with an intense light at the transmitter, the transmitter frequency may be 10,030 kc.; and with no light 10,020 kc., a difference of 10 kc., or 1% of the meanv value. By using a local beat oscillator of 10,000 kc., the resulting beat frequency is 20,000 cycles for no illumination and 30,000 cycles for intense illumination.

The beat frequencies existing in the resistor 30 are transmitted through the radio frequency choke 31 and the stopping condenser 32 to produce the intermediate frequency voltages across the resistor 33. This voltage is amplified by the thermionic devices 34 and 35 and their associated circuits in any suitable manner, as many stages of amplification being supplied as is necessary. The amplificationl characteristic of these ampliers are so determined that they will amplify in a practically uniform manner throughout the frequency range. That is, there will be as much amplification at 20,000 cycles as at 30,000 cycles. This may be adjusted by making the stopping condensers in the plate grid coupling elements of sufiiciently high capacity.

A current limiting device. 38 consisting of a two electrode vacuum tube is provided across the grid filament of the tube 37.' This is for the purpose of obtaining substantially constant voltage across the input of the tube 37 independent of amplitude variations caused by fading of the signals between the transmitter and receiver. This also will permit slight variations in the amplification of the amplifier devices 34 and 35 without a resultant amplitude variation of the frequency being supplied to the device 37. The adjustment is made so that the voltage on the grid of the thermionic device 37 at the lowest point of combined fading and amplification will be approximately in which Ec is the bias voltage. Then at the high signal strengths, the device 36 will prevent more than this minimum voltage being impressed across the tube 37. Consequently, the receiver corrects for any fading that occurs and the fre`- quency alone is an indication as to whether a high or low intensity response is desired.

The differentiation between high and low intensities is made by a filler amplifier. Thus the stopping condensers of the Asucceeding amplification stages comprising the thermionic devices 38 and 39 and the output transformer are so designed with respect to the associated apparatus that the output voltage across the output transformer adapted to supply the light will be the largest at 30 kc., and the smallest at 20 kc. with the grid voltage of the device 37 the same for all frequencies. `Thus the light 40 will be the most intense whenever the input light on the photoelectric cell of Fig. 1 or 2 is strong, and weak whenever the input light is weak.

Having thus described my invention, I will now briefly describe its operation. The varying light falling upon the photo-electric cell of either Figs. l or 2 produces a current which is amplified and is adapted to control the frequency of a high radio frequency oscillator. In Fig. 1l this is accomplished by supplying the amplified pulses to the mechanical devices adapted to vary the capacity of the condenser compcsed of the movable plate 15 and the stationary plate 16. This condenser acts in addition to the frequency controlling condenser of the high frequency oscillator and thus varies the frequency of the oscillations, in accordance with light intensity. In Fig. 2 the amplified electrical impulses adapted to vary the voltage impressed upon the grid and plate circuits of the oscillator in such a manner that the frequency of the oscillator will be varied, but the strength of the oscillation will remain constant. Thus the oscillator 25 of Fig. 2 will radiate through appropriate amplifying devices at a frequency which will vary in accordance with the variations in light intensity. The receiver is adapted to receive all of the transmitted frequencies'with equal intensity and the incoming frequencies are all detected by the well-known heterodyne method. The heterodyning frequencies produced will thus vary in accordance with the light intensities at the transmitter and this varying frequency is so amplied as to produce a substantially constant amplitude with varying frequencies. TheNarying frequency is then impressed upon an yamplifier adapted to produce a varying intensity current and the current limiting device 36prevents any uctuation in the input intensity of said last mentioned amplifier. The characteristic of the last amplifier is such that the amplification will be great at one end of the frequency band which is adapted to be amplied by it, and little at the other end of said' band. In other words, the frequency modulated input is amplitude modulated by said amplifier. The varying amplitude varying frequency current is supplied to a light 'device or any other utilization circuit and the energy thus supplied varies in the same manner as that supplied by the original photo-cell at the transmitter.

Having thus described my invention, it is to be understood that I am not to be limited by the' form of my invention as set forth in the foregoing specification and in the drawing, for the purpose of illustration only, but rather by the. scope of my invention as defined by the appended claims.

I claim:

l. The method cf transmission of intelligence which comprises producing a low frequency representing the intelligence to be transmitted, producing a high frequency, amplifying the low frequency, varying the frequency of the high frequency in accordance with said amplified low frequency, maintaining a constant high frequency intensity, radiating the frequency modulated highl frequency thus produced, receiving the frequency modulated signals, producing a high frequency, heterodyning the incoming frequency with said locally produced frequency, amplifying the varying beat frequency produced thereby while maintaining substantially constant intensity regardless of the frequency of said beat, controlling the intensity of the amplified beat, again amplifying said varying frequency beat current in such a manner that the current at one endof the varying beat frequency range will be amplified a greater amount than at the other end of the same range, and utilizing the varying intensity energy thus produced.

2. 'I'he method of transmission and reception of varying light intensities whichcomprises producing a low frequency, varying the amplitude of the low frequency in accordance with the light fluctuations to be transmitted, producing a high frequency, amplifying the iiuctuating intensity low frequency, varying the frequency of the high frequency in accordance with said amplitude varied low frequency, maintaining a constant high frequency intensity, radiating the frequency modulated high frequency thus produced, receiving the frequency modulated signals, producing a high frequency, heterodyning the incoming frequency with said locally produced frequency, amplifying the varying beat frequency produced by said hetercdyning while maintaining substantially constant intensity regardless of the frequency of said beat, controlling the intensity of the amplified beat, again amplifying said varying frequency beat current in such amanner that the current at one end of the varying beat frequency range will be amplified a greater `amount than at the other end of the same range, and utilizing the varying -intensity energy thus produced to control the intensity of a light.

3. Apparatus for the transmission and reception of intelligence which comprises means for producing a high radio frequency, means for producing a low frequency to be transmitted, means for varying the frequency'of said high frequency in accordance with the intensity of said low frequency, means for radiating the frequency modulated high frequency thus produced, means for receiving said last mentioned frequency, means for locally producing a high radio frequency, means for combining said incoming frequency and said locally produced frequency to produce a varying frequency beat, means for amplifying said beat frequency while maintaining constant amplitude, means for limiting the amount of said amplified beat frequency current, means for again amplifying the amplified varying beat frequency as thus limited, said last mentioned means comprising filter means for amplifying the varying beat frequency at one end of the frequency range more than at the other end, and means for utilizing the varying current thus produced.

4. Apparatus for the transmission and reception of intelligence which comprises means for producing a high radio frequency, means for pro-` ducing a low frequency to be transmitted, means for varying the frequency of said high frequency in accordance with the intensity of said low frequency, means for radiating the frequency modulated high frequency thus produced, means for receiving said high frequency, means for locally producing a high radio frequency, means for combining said incoming frequency and said locally produced frequency to produce a Varying frequency beat means for limiting the amount of said beat frequency current, means for amplifying the varying beat frequency as thus limited, said last mentioned means comprising filter means for amplifying the varying beat frequency at one end of the frequency range more than at the other end, and means for utilizing the varying current thus produced.

5. Apparatus for the transmission and recepbeat frequency current which is substantially less than the maximum value of beat frequency current after amplification thereof so as to maintain a substantially constant intensity of beat frequency energy despite adverse transmission conditions, means for amplifying the varying beat frequency as thus limited. said last mentioned means comprising a filter amplifier device arrangedso as to amplify the varying beat frequency at one end of the frequency range more than at the other end and means for utilizing the varying current thus produced.

JOHN Hays HAMMOND, Jn. 

